1. Technical Field
The present invention relates generally to plain bearings and to their method of manufacturing and particularly to multilayer engine bearings of the type having a metal backing on which a functional bearing layer is applied which in turn is plated with an overlay of soft metal.
2. Related Prior Art
Plain engine journal-type bearings for use in high load engine applications for journaling crank shafts and the like typically include a base lining member having a functional bearing layer of either copper-lead or aluminum alloy applied to a steel backing. A single layer of a lead-tin-copper alloy having a thickness of about 25 .mu.m is typically plated onto the functional layer. Often, a nickel diffision barrier or copper bonding layer is interposed between the overplate and functional layer to prevent the tin from diffusing from the overplate into the bearing layer. As a final step, the bearing is typically coated with a micro-thin layer of tin or lead-tin flash plating having a thickness of about 1 .mu.m or less. The flash plating is primarily cosmetic, giving the product a bright, pleasing appearance. It also provides a level of corrosion protection to the steel backing. Within a short time during engine break-in, the micro-thin flash plating applied to the running surface of the bearing dissipates.
In service, such multilayer crank shaft bearings are subjected to high dynamic loads that vary in magnitude and direction due to the inertial loads applied by the piston and connecting rod mechanism and by the cylinder gas. The soft overplate layer enables such bearings to conform under high load forces to any misalignments or changes in profile of loading of the member being journaled, so that the loads are distributed across a greater surface area of the bearing. The overplate also allows any foreign particles of dirt or metal that may come between the bearing surface and the member being journaled to become embedded or absorbed into the bearing surface, so as to protect the bearing journal from excessive wear or damage.
It is generally accepted that conformability and embedibility of the overplate are dependent upon overplate thickness, with a thicker overplate being preferred. It is also generally known that as the thickness of the overplate increases, so does the susceptibility to bearing fatigue (i.e., the fracturing of the bearing surface when under load). Resistance to fatigue cracking requires that the bearing surface possess sufficient tensile strength to enable it to undergo minor configuration changes without cracking. Thus, it is necessary to balance the competing properties of conformability and embedibility with fatigue resistance when designing engine bearings, particularly ones that are subjected to high dynamic loading.
For many high load engine applications, it has been found that the 25 .mu.m thick single layer lead-tin-copper overplate mentioned above provides excellent conformability and embedibility characteristics and good fatigue resistance. However, as the output and efficiency of engines increases, so does the dynamic loading placed on the crank shaft bearings, and thus the increased potential for bearing fatigue. Under extreme loading conditions, the conventional bearings described above employing the single 25 .mu.m thick overplate of lead-tin-copper are prone to fatigue. Efforts to alleviate fatigue by simply decreasing the thickness of the single layer overplate to less than 25 .mu.m have been generally unsuccessful, as it is at the cost of sacrificing acceptable levels of conformability and embedibility. Thus, there exists a need in the industry for an improved journal bearing that can perform under such extreme high dynamic loading conditions without experiencing fatigue and while maintaining good conformability and embedibility characteristics.